Process for forming polycrystalline silicon layer by laser crystallization

ABSTRACT

A process for forming a polycrystalline silicon layer includes the following steps. Firstly, at least one seed is formed on a substrate. Then, an amorphous silicon layer is formed on the substrate and overlies the seed. Then, the amorphous silicon layer is irradiated with a laser to melt the amorphous silicon layer. Afterward, the molten amorphous silicon layer is recrystallized to form a polycrystalline silicon layer.

FIELD OF THE INVENTION

The present invention relates to a process for forming a polycrystallinesilicon layer, and more particularly to a process for forming apolycrystalline silicon layer with large crystal grain size and highuniformity by laser crystallization.

BACKGROUND OF THE INVENTION

Polycrystalline silicon thin film transistors are commonly used as basicelectronic devices for controlling pixels of active matrix liquidcrystal displays (AMLCDs) and active matrix organic light emittingdisplays (AMOLEDs). In addition, the polycrystalline silicon thin filmtransistors are also used as basic electronic devices required by theperipheral driving circuits and/or control circuits in these displays.

During production of the polycrystalline silicon thin film transistors(TFTs), the crystallization procedure for forming a polycrystallinesilicon layer is critical. The electrical properties and uniformity ofthe polycrystalline silicon thin film transistors are primarilydetermined by this procedure.

A conventional process for forming a polycrystalline silicon layer usinga crystallization method is illustrated with reference to FIGS. 1( a)and 1(b). Firstly, an amorphous silicon layer 104 is formed on a glassor plastic substrate 100. Then, the amorphous silicon layer 104 isirradiated with an excimer laser to be molten. The molten amorphoussilicon layer is cooled and recrystallized into a polycrystallinesilicon layer 105.

Referring to FIG. 2, the relationship between the energy density of anexcimer laser irradiating on the amorphous silicon layer and theelectrical property such as the electron mobility of the resultingrecrystallized polycrystalline silicon layer is shown. As shown, theenergy density effective for forming a polycrystalline silicon layerwith good electrical property is confined to a narrow range. Anexcessive energy density may result in microcrystalline silicon ratherthan polycrystalline silicon. The electrical property ofmicrocrystalline silicon is different from that of the polycrystallinesilicon. For example, its electron mobility is not as high as requiredin some applications. On the other hand, an insufficient energy densitysubstantially fails to effectively melt the amorphous silicon forsubsequent recrystallization.

Furthermore, due to the variability of the excimer laser, the grain sizeof the recrystallized polycrystalline silicon layer 105 may be too smalland non-uniform, and thus the electron property of the thin filmtransistor product varies significantly.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forforming a polycrystalline silicon layer consisting of relatively uniformand large crystal grains.

It is another object of the present invention to provide a process forforming a polycrystalline silicon layer with a less criticalenergy-density range of an pulse laser for the crystallization of thepolycrystalline silicon layer.

In accordance with a first aspect of the present invention, there isprovided a process for forming a polycrystalline silicon layer. Firstly,at least one seed is formed on a substrate. Then, an amorphous siliconlayer is formed on the substrate and overlies the seed. Then, theamorphous silicon layer is irradiated with a laser to melt the amorphoussilicon layer. Afterward, the molten amorphous silicon layer isrecrystallized to form a polycrystalline silicon layer.

In one embodiment, the step of forming the at least one seed on thesubstrate comprises sub-steps of forming an intermediate covering layeron the substrate, patterning the intermediate covering layer to definethe intermediate covering layer as a specified pattern, forming anamorphous silicon spacer beside the specified pattern, and removing thespecified pattern with the spacer remained.

For example, the substrate is a glass substrate or a plastic substrate,the laser is an excimer laser, and the intermediate covering layer ismade of silicon nitride or metal.

In accordance with a second aspect of the present invention, there isprovided a process for forming a polycrystalline silicon layer. Firstly,a first region and a second region on a substrate are defined. Then, atleast one seed on the first region of the substrate is formed. Then, anamorphous silicon layer is formed on the first and the second regions ofthe substrate. Then, the amorphous silicon layer is irradiated with alaser to melt the amorphous silicon layer. Afterward, the moltenamorphous silicon layer is recrystallized on the first region to form apolycrystalline silicon layer.

If necessary, the process can further comprise a step of recrystallizingthe molten amorphous silicon layer on the second region to form amicrocrystalline silicon layer.

In accordance with a third aspect of the present invention, there isprovided a process for fabricating a polycrystalline silicon layer.Firstly, a substrate is provided. Then, an intermediate covering layeris formed on the substrate. Then, the intermediate covering layer ispatterned to define the intermediate covering layer as a specifiedpattern. Then, an amorphous silicon spacer is formed beside thespecified pattern. Then, the specified pattern with the spacer remainedis removed to form at least one seed the substrate. An amorphous siliconlayer is then formed on the substrate and overlies the seed. Then, theamorphous silicon layer is irradiated with a laser to melt the amorphoussilicon layer. Afterward, the molten amorphous silicon layer isrecrystallized to form a polycrystalline silicon layer.

Accordingly, the present invention will become more readily apparent tothose ordinarily skilled in the art after reviewing the followingdetailed description and accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) and 1(b) are schematic cross-sectional diagrams illustratinga conventional process for forming a polycrystalline silicon layer byusing a crystallization method;

FIG. 2 is a plot showing the electron mobility change and phase changewith the energy density of an excimer laser irradiating on the amorphoussilicon layer according to prior art;

FIGS. 3( a) to 3(i) are schematic cross-sectional diagrams illustratinga process for forming a polycrystalline silicon layer of a thin filmtransistor according to an embodiment of the present invention;

FIG. 4( a) is a top view of a polycrystalline silicon layer formedaccording to the process of the present invention;

FIG. 4( b) is a top view of the polycrystalline silicon layer of FIG. 4(a) formed thin film transistors;

FIG. 5 is a plot showing the electron mobility change and phase changewith the energy density of a pulsed laser irradiating on the amorphoussilicon layer according to the present invention; and

FIG. 6 is a schematic diagram illustrating the simultaneous formation ofpolycrystalline silicon and microcrystalline silicon regions to obtaintwo kinds of thin film transistors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A process for forming a polycrystalline silicon layer of a thin filmtransistor is illustrated with reference to FIGS. 3( a) to 3(i).

In FIG. 3( a), an intermediate covering layer 204, for example made ofsilicon nitride (SiNx) or metal, is formed on a substrate 200 such as aglass or a plastic substrate.

Then, as shown in FIG. 3( b), a photoresist 206 is formed on theintermediate covering layer 204 and properly patterned to cover selectedregions of the intermediate covering layer 204. The intermediatecovering layer 204 is then etched with the photoresist 206 serving as amask. After the etching procedure is completed and the photoresist 206is removed, a specified pattern of the intermediate covering layer 204is obtained, as can be seen in FIG. 3( c).

In FIG. 3( d), an amorphous silicon layer 208 is then formed on theresulting structure of FIG. 3( c). An anisotropic etching procedure issubsequently performed on the amorphous silicon layer 208, therebyforming amorphous spacers 210 beside the specified pattern of theintermediate covering layer 204, as can be seen in FIG. 3( e).

Then, as shown in FIG. 3( f), the specified pattern of the intermediatecovering layer 204 is removed, but the amorphous spacers 210 remain onthe substrate 200. The remaining amorphous spacers 210 are used as seedsfor the following crystallization procedure.

In FIG. 3( g), additional amorphous silicon layer 212 is formed on thesubstrate 200 and overlies the seeds 210. Then, as shown in FIG. 3( h),the amorphous silicon layer 212 is irradiated with a pulse laser (forexample an excimer laser) to melt the amorphous silicon layer 212. Bycontrolling energy density of the pulse laser, the amorphous siliconlayer 212 can be fully melted. Meanwhile, the seeds 210 remaining on thesubstrate 200 will not be fully molten. In such manner, when the moltenamorphous silicon layer 212 is cooled, the seeds 210 will promoteformation of relatively uniform and large crystal grains along bothsides of the seeds (in the arrow directions). Thus, the molten amorphoussilicon layer 212 is recrystallized to form a polycrystalline siliconlayer 212 a as shown in FIG. 3( i), and grain boundaries 214 are formedin the polycrystalline silicon layer 212 a.

The polycrystalline silicon layer formed according to the process of thepresent invention is illustrated in FIG. 4( a). In FIG. 4( a), theregions between two dotted lines indicate the location of the seeds 210,while the solid lines indicate the grain boundaries 214, respectively.Afterwards, thin film transistors are formed by any of suitablemanufacturing processes, which are not intended to be describedredundantly herein. In FIG. 4( b), the hatched portions indicate thelocation of the thin film transistors, and the source electrodes S, thegate electrodes G, the drain electrodes D and thus channels of the thinfilm transistors are shown.

As previously described, the energy density of the pulse laser (forexample an excimer laser) effective for forming a polycrystallinesilicon layer with good electrical property is confined to a narrowrange. The improper energy density is likely to result in the formationof microcrystalline silicon instead of desired polycrystalline siliconor result in non-uniform grain size of the recrystallizedpolycrystalline silicon layer. According to the present invention, seedsare additionally provided between the substrate and the amorphoussilicon layer prior to the formation of the amorphous silicon layer.Accordingly, when a certain energy density of excimer laser required formelting the amorphous silicon layer in the prior art, or even if alittle more intense excimer, is applied to the amorphous silicon layerof the present invention for the same period of time, the seeds areprotected from being molten away. In other words, the energy-densityrange of an excimer laser suitable for melting and recrystallizing theamorphous silicon layer into the polysillicon layer can be broadened.Referring to FIG. 5, the relationship between the energy density of apulse laser irradiating on the amorphous silicon layer and theelectrical property such as the electron mobility of the resultingrecrystallized polycrystalline silicon layer is shown. As shown, theenergy density effective for forming a polycrystalline silicon layerwith good electrical property is broadened. After the amorphous siliconlayer is fully melted and then cooled, the molten amorphous siliconlayer is recrystallized starting from sides of the seeds so as to form apolycrystalline silicon layer with relatively uniform and large crystalgrains.

In practice, while some thin film transistors require gate channelsformed of polycrystalline silicon layers, others require gate channelsmade of microcrystalline silicon layers. Sometimes, two types of thinfilm transistors respectively with these two gate channels are requiredto be formed on the same glass substrate. Conventionally, these twotypes of thin film transistors have to be separately manufactured. Suchconventional process is complicated and has unsatisfactory reliability.According to the present invention, these two types of transistors canbe formed on the glass substrate simultaneously so as to reducefabrication cost. Referring to FIG. 6, it is assumed that a first region220 and a second region 240 on a glass substrate are to be formedthereon two types of transistors with polycrystalline silicon andmicrocrystalline silicon gate channels, respectively. Seeds are firstformed on the first region 220 in a manner as described in the aboveembodiment, but no seeds are formed on the second region 240. Then, anamorphous silicon layer (not shown) is formed on and overlies the firstregion 220 and the second region 240. The amorphous silicon layer isirradiated with a sufficient intensity of pulse laser to be completelymolten. Due to the presence of the remaining seeds after the meltingprocedure in the first region 220, a polycrystalline silicon layer withrelatively uniform and large crystal grains is formed on the firstregion 220 in the subsequent cooling procedure. Meanwhile, amicrocrystalline silicon layer is formed in the second region 240.

From the above description, it is understood that the process forforming a polycrystalline silicon layer by burying seeds under theamorphous silicon layer in advance can make the condition of the appliedpulse laser less critical so as to easily obtain a polycrystallinesilicon layer with relatively uniform and large crystal grains and thusthin film transistor products with stable electron properties.

While the invention has been described in terms of what is presentlyconsidered to be the most practical and preferred embodiments, it is tobe understood that the invention needs not be limited to the disclosedembodiment. On the contrary, it is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the appended claims which are to be accorded with the broadestinterpretation so as to encompass all such modifications and similarstructures.

1. A process for forming a polycrystalline silicon layer, comprisingsteps of: forming at least one seed made of amorphous silicon on asubstrate; forming an amorphous silicon layer on said substrate,overlying said amorphous silicon seed; irradiating said amorphoussilicon layer with a laser to melt said amorphous silicon layer; andrecrystallizing said molten amorphous silicon layer to form apolycrystalline silicon layer; wherein said step of forming said atleast one seed on said substrate comprises sub-steps of: forming anintermediate covering layer on said substrate; patterning saidintermediate covering layer to define said intermediate covering layeras a specified pattern; forming an amorphous silicon spacer beside saidspecified pattern; and removing said specified pattern with said spacerremained.
 2. The process according to claim 1 wherein said intermediatecovering layer is made of silicon nitride.
 3. The process according toclaim 1 wherein said intermediate covering layer is made of metal.
 4. Aprocess for forming a polycrystalline silicon layer, comprising stepsof: defining a first region and a second region on a surface of asubstrate; forming an intermediate covering layer on said first regionof said substrate; patterning said intermediate covering layer to definesaid intermediate covering layer as a specified pattern; forming anamorphous silicon spacer beside said specified pattern; removing saidspecified pattern with said spacer remained to form at least one seed onsaid first region of said substrate; forming an amorphous silicon layeron said first and said second regions of said substrate; irradiatingsaid amorphous silicon layer with a laser to melt said amorphous siliconlayer; and recrystallizing said molten amorphous silicon layer on saidfirst region to form a polycrystalline silicon layer.
 5. The processaccording to claim 4 wherein said substrate is a glass substrate.
 6. Theprocess according to claim 4 wherein said substrate is a plasticsubstrate.
 7. The process according to claim 4 wherein said laser is anexcimer laser.
 8. The process according to claim 4 wherein saidintermediate covering layer is made of silicon nitride.
 9. The processaccording to claim 4 wherein said intermediate covering layer is made ofmetal.
 10. The process according to claim 4 further comprising a step ofrecrystallizing said molten amorphous silicon layer on said secondregion to form a microcrystalline silicon layer.
 11. A process forfabricating a polycrystalline silicon layer, comprising steps of:providing a substrate; forming an intermediate covering layer on saidsubstrate; patterning said intermediate covering layer to define saidintermediate covering layer as a specified pattern; forming an amorphoussilicon spacer beside said specified pattern; removing said specifiedpattern with said spacer remained to form at least one seed on saidsubstrate; forming an amorphous silicon layer on said substrate,overlying said seed; irradiating said amorphous silicon layer with alaser to melt said amorphous silicon layer; and recrystallizing saidmolten amorphous silicon layer to form a polycrystalline silicon layer.12. The process according to claim 11 wherein said substrate is a glasssubstrate.
 13. The process according to claim 11 wherein said substrateis a plastic substrate.
 14. The process according to claim 11 whereinsaid laser is an excimer laser.
 15. The process according to claim 11wherein said intermediate covering layer is made of one of siliconnitride and metal.
 16. A process for forming a polycrystalline siliconlayer, comprising steps of: defining a first region and a second regionon a surface of a substrate; forming at least one seed on said firstregion of said substrate; forming an amorphous silicon layer on saidfirst and said second regions of said substrate; irradiating saidamorphous silicon layer on said first region and said second region witha laser to melt said amorphous silicon layer; and recrystallizing saidmolten amorphous silicon layer on said first region and said secondregion to form a polycrystalline silicon layer and a microcrystallinesilicon layer, respectively.